This project is concerned with the effect of thyroid hormone on gonadotrophin secretion. We have shown that thyroid hormone can attenuate the post-castration rise of LH, as well as the spontaneous and estrogen-induced preovulatory surges of LH. These effects cannot be ascribed to interference with the radioimmunological measurement of LH, nor to alteration of the molecular size and metabolic clearance rate of LH. Furthermore, since thyroid hormone attenuated the post-castration rise of LH, these effects could not be ascribed to alterations in estrogen metabolism. Consequently, we are now examining the hypothalamus and pituitary as sites at which thyroid hormone modulates gonadotropin secretion. BIBLIOGRAPHIC REFERENCES: Freeman, M.E., F.T. LaRochelle, Jr., and R.B. Moore. 1976. Regulation of the preovulatory surge of luteinizing hormone by thyroxine. Endocrinology: in press. Freeman, M.E., K.C. Dupke, and C.M. Croteau. 1976. Extinction of the estrogen-induced daily signal for LH release in the rat: A role for the proestrous surge of progesterone. Endocrinology: in press.